LAMELLIBRANCHIATA. ill 



the base as a broad, undefined depression. Umbonal slope obtusely angular, 

 extending to the post-inferior extremity. Post-cardinal slope wide, marked 

 along the middle of its length by a distant fold. 



Surface marked by fine concentric strige which, on the posterior part of the 

 shell, are somewhat fascicled. Surface also marked by fine, equal radiat- 

 ing striae, giving the surface a somewhat cancellated appearance. The radii 

 are obscure on the post-cardinal slope and are principally developed in the 

 umbonal depression. 



The specimen described has a length of 39 mm. and a height, at the pos- 

 terior extremity, of 18 mm. 



This species somewhat resembles P. sectifrcms, but is broader and more 

 abruptly truncate at the posterior end, and the umbonal ridge is more distinctly 

 defined. 



Formation and locality. In the Chemung group, south-east of Panama, N. Y. 



Phthonia nitida, n. sp. 



PLATE XCIV, FIGS. 6, 6. 



Shell below the medium size, narrow-elliptical ; length more than twice the 

 greatest height ; basal margin nearly straight, gently curving upward at the 

 anterior and more rapidly at the posterior. Posterior extremity rounded, a 

 little oblique above. Cardinal line nearly straight, or slightly arcuate. 

 Anterior end short, naiTOwly rounded. 



Valves moderately convex below and posteriorly, becoming gibbous in the 

 middle and umbonal region. 



Beaks sub-anterior, small, incurved and appressed. Umbo flattened, and 

 this feature is continued, as a broad undefined depression, to the basal margin. 

 Umbonal slope convex, not defined, merging into the general convexity 

 before reaching the post-inferior extremity. 



Surface marked by fine, filiform concentric striae, which sometimes become 

 fasciculate on the posterior and basal portions of the shell ; and by fine radi- 

 ating striae extending from the beaks over the entire surface, being stronger 

 61 



